Low noise amplifier for ultra wide band

ABSTRACT

A low noise amplifier (LNA) for ultra wide band receives and amplifies identical RF signals in different frequency bands, and includes more than one pair of narrow band LNAs coupled in parallel, and a load circuit which increases load impedance of the entire circuit of the narrow band LNAs. The LNA can not only amplify the RF signal in the UWB but also obtain the low noise and the high gain that are features of the conventional narrow band LNA.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(a) from KoreanPatent Application No. 2005-9135 filed on Feb. 1, 2005 in the KoreanIntellectual Property Office, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Apparatuses consistent with the present invention relate to a low noiseamplifier for ultra wide band. More particularly, the present inventionrelates to a low noise amplifier (LNA) for ultra wide band (UWB), whichamplifies signals in the ultra wide band by coupling narrow band LNAsand thus obtains high gain and low noise figure (NF) which arecharacteristics of the narrow band LNA.

2. Description of the Related Art

With the wide use of Internet and the rapid increase of multimedia data,there is a great demand for very high speed communication networks.Among those networks, a local area network (LAN) has been introduced inthe late 1980's. The transfer rate of the LAN was about 1-4 Mbps at theinitial phase and has increased to 100 Mbps. As there is an increasingdemand for global access to networks due to the prevalence of portablecomputers and personal digital assistants (PDAs), the wirelesscommunication technology is of great concern.

To respond to those demands, an ultra wide band communication techniquewhich enables wireless communications in high speed wide band togetherwith existing wireless communication services is under development. Todistinguish the UWB from the existing narrow band systems and the wideband systems with 3G cellular technology, the UWB is defined as awireless transmission technology with bandwidth that occupies more than25% or 1.5 GHz of center frequencies. Primarily, the UWB occupies thefrequency band ranging from 3.1 to 10.6 GHz and covers a transmissionrange of 10 meters through 100 meters.

A wireless terminal used for such wireless communications, as shown inFIG. 1, includes a receiving block 10, a transmitting block 20, acontroller 30, a modem 40, and a local oscillator (LO) 50.

The receiving block 10 includes a low noise amplifier (LNA) 11, a mixer13, and a filter 15. A radio frequency (RF) signal, which is a highfrequency signal received via an antenna, has a very low power level dueto attenuation and noise. The LNA 11 removes the noise from the RFsignal and amplifies the RF signal. The mixer 13 converts the amplifiedRF signal to an intermediate frequency (IF) signal. The filter 15filters a required region from the IF signal and provides the filteredregion to the modem 40.

The modem 40 converts the IF signal input from the filter 15 to a Rxbaseband signal. The modem 40 also converts a Tx baseband signal to beprovided to the filter 25 of the transmitting block 20 to an IF signal.The LO 50 receives a control signal and generates LO signals to be fedto the modem 40 and the filter 25.

The transmitting block 20 includes a filter 25, a mixer 23, and a poweramplifier 21. The filter 25 filters a required region from the IF signalto be transmitted. The mixer 23 converts the IF signal to the RF signal.The power amplifier 21 amplifies the output so that the RF signal can betransmitted.

The sensitivity of the wireless terminal directly depends on a noisefigure (NF), and substantially, noise of the receiving end is determinedby the LNA 11. Thus, it is crucial to design the LNA 11 with suitablelinearity and gain while the noise is minimized. The related art candesign the LNA 11 to meet such requirements in the narrow band mostlyused.

With the active development and research on UWB, various LNA designshave been made to implement the LNA available in the UWB.

U.S. Pat. No. 6,735,421 (hereafter, referred to as a prior art 1)titled, “Receiver Including Low Noise Amplifier And FrequencyDown-Converter For A Wireless Telecommunication System,” describes anarrow band LNA including a field effect transistor (FET), a resistor R3and a capacitor C1 for the negative feedback connected in parallelbetween a gate terminal and a drain terminal of the FET as shown in FIG.2. The resistor R3 and the capacitor C1 reduce the gain in the lowfrequency and prevent the amplifier from getting high gain in the lowfrequency. In other words, the RC feedback design increases the outputwith respect to the input of the LNA and expands the bandwidth ratherthan reducing the gain. As a result, the prior art 1 can realize the LNAfor the wide band but produces high NF and low gain.

U.S. Pat. No. 6,806,777 hereafter, referred to as a prior art 2 titled“Ultra Wide Band Low Noise Amplifier And Method” discloses a wide bandLNA using a common-gate input as shown in FIG. 3. The LNA of the priorart 2 includes a cascode 102 with a load tracking (LT) network 104, anda common-gate part 106. The common-gate part 106 is in a common-gateamplifier arrangement since an RF signal is input to its source and itsgate is AC grounded. Hence, the wide band amplifier can be designedsince an RFin terminal can match the wide band impedance. However, theLNA of the prior art 2 cannot obtain high gain due to the impedance butproduces high NF as the RF signal is input to the source.

Therefore, what is needed is a LNA for the UWB with the high gain andlow noise advantages of the narrow band LNA.

SUMMARY OF THE INVENTION

Embodiments of the present invention have been provided to address theabove-mentioned and other problems and disadvantages occurring in theconventional arrangement, and aspects of these embodiments provide a lownoise amplifier (LNA) for ultra wide band (UWB) with high gain and lownoise by use of narrow band LNAs.

These aspects are achieved by providing a low noise amplifier (LNA) forultra wide band (UWB) including more than one pair of narrow band LNAswhich are coupled in parallel, and receive and amplify identical RFsignals in different frequency bands; and a load circuit which increasesthe load impedance of the entire circuit of the narrow band LNAs.

The narrow band LNAs each may include an amplifying element and a pairof inductors which are connected to a source and a gate, respectively,of the amplifying element.

The amplifying element of each narrow band LNA may be connected to acommon-gate amplifying element forming a cascode amplifier.

The load circuit may be positioned between the common-gate amplifyingelements, and may include a load resistor, an inductor, and a capacitorthat are connected in parallel.

The low noise amplifier may further include a buffer amplifying elementwhich may include the load circuit between a gate and a drain, tominimize the influence of external impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofexemplary embodiments, taken in conjunction with the accompanyingdrawing figures of which:

FIG. 1 is a schematic block diagram of a conventional wireless terminal;

FIG. 2 is a circuit diagram of a conventional wide band LNA used for awireless communication system;

FIG. 3 is a circuit diagram of a conventional wide band LNA using acommon-gate input;

FIG. 4 is a schematic circuit diagram of a conventional narrow band LNA;

FIG. 5 is a schematic circuit diagram of a LNA for UWB according to anembodiment of the present invention; and

FIG. 6 is a frequency response graph of an RF signal amplified by theLNA of FIG. 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Certain exemplary embodiments of the present invention will now bedescribed in greater detail with reference to the accompanying drawings.

In the following description, same drawing reference numerals are usedfor the same elements even in different drawings. The matters defined inthe description, such as detailed construction and element descriptions,are provided to assist in a comprehensive understanding of theinvention. Also, well-known functions or constructions are not describedin detail since they would obscure the invention in unnecessary detail.

A general narrow band low noise amplifier (LNA), as shown in FIG. 4,employs inductors at a source and a gate of an amplifying element andthus generates an impedance input to the LNA up to 50Ω.

A LNA for ultra wide band (UWB) according to an embodiment of thepresent invention is designed by coupling narrow band LNAs in parallel.

FIG. 5 is a schematic circuit diagram of the LNA for the UWB accordingto an embodiment of the present invention. As shown in FIG. 5, the LNAincludes first and second amplification parts 110 and 120 which arenarrow band amplification parts arranged in parallel.

The first amplification part 110 may include an amplifying element M1,and a pair of inductors L_(G1) and L_(S1) connected to a source and agate of the amplifying element M1. The second amplification part 120 mayinclude an amplifying element M2, and a pair of inductors L_(G2) andL_(S2) connected to a source and a gate of the amplifying element M2.

The amplifying elements M1 and M2 of the first and second amplificationparts 110 and 120 amplify a radio frequency (RF) signal input via anantenna. As the amplifying elements M1 and M2 are coupled in parallel,the input RF signal is divided to them and amplified in the respectivefirst and second amplification parts 110 and 120. The amplifying elementM1 may be designed in a size different from the amplifying element M2.As a result, the frequencies of the amplified signals are different fromeach other even when the identical RF signals are input.

Generally, the impedance of an RF signal input to a narrow band LNA is50Ω. For the sake of impedance matching, the inductances of theinductors L_(G1) and L_(S1) connected to the amplifying element M1 maybe set such that the first amplification part 110 has an input impedanceof 50Ω at or near its center frequency, and the inductances of theinductors L_(G2) and L_(S2) connected to the amplifying element M2 maybe set such that the second amplification part 120 has an inputimpedance of 50Ω at or near its center frequency. In other words, theinductances of the respective inductors may be set to be suitable to thedifferent sizes of the amplifying elements M1 and M2 so that at or nearthe frequencies of the RF signal amplified at the amplifying elements M1and M2 having the different sizes, the RF signal impedance matches orapproaches the input impedances of the first amplification part 110 andthe second amplification part 120.

Amplifying elements M3 and M4 connected to the drains of the amplifyingelements M1 and M2 of the first and second amplification parts 110 and120 respectively, may be common-gate amplifying elements coupled inparallel and having the same input. The amplifying elements M3 and M4may be realized by NMOS transistors. The amplifying elements M1 and M3form a cascode amplifier, and the amplifying elements M2 and M4 form acascode amplifier. In practice, the cascode amplifier can improve theamplification characteristics. Thus, the design of the amplifyingelements M3 and M4 can increase the gain of the first and secondamplification parts 110 and 120, expand the bandwidth, and reduce thenoise.

A load circuit is designed between a gate and a drain of a bufferamplifying element M5, to improve the gain by increasing the impedanceof the entire circuit. The load circuit may include a resistor Ro, aninductor Lo, and a capacitor Co that are connected in parallel. In theembodiment of the present invention, the LNA may employ only one loadcircuit because the resistor Ro, the inductor Lo, and the capacitor Coshare the output of the first and second amplification parts 110 and 120by means of the common-gate amplifying elements M3 and M4.

The buffer amplifying element M5 connected to the load circuit has highinput impedance and low output impedance, and serves to forward thevoltage input to the gate to the source. As the buffer amplifyingelement M5 converts the voltage, the impedance at the load circuit canminimize effects on the entire UWB LNA by the characteristic change dueto the connection of an external circuit, and thus the deterioration ofthe characteristic of the UWB LNA can be prevented.

I_(B) which is an ideal current source for supplying a constant powerregardless of the voltage, is connected to the buffer amplifying elementM5.

As designed above, how the UWB LNA amplifies the RF signal is nowexplained.

When the RF signal having the center frequency f in the wide band isinput to RFin of the UWB LNA, the RF signal is divided and input to thefirst and second amplification parts 110 and 120 respectively. The inputRF signals are amplified while passing through the amplifying elementsM1 and M2 having the different sizes. The first and second amplificationparts 110 and 120 amplify center frequency bands f₁ and f₂ adjacent tothe input center frequency f. The amplified RF signals in the centerfrequency bands f₁ and f₂ pass through the load circuit where thefrequency bands are combined and the voltage is amplified. Next, the RFsignal is buffered at the buffer amplifying element M5 and output toRFout.

The RF signal is amplified in the center frequency bands f₁ and f₂ atthe first and second amplification parts 110 and 120 as shown in FIG. 6.Hence, the wide band LNA can obtain the bandwidth more than two times aswide as the conventional narrow band LNA.

As set forth above, in the UWB LNA, the pair of the narrow LNAs iscoupled in parallel and the amplifying elements M1 and M2 of the narrowband LNAs are designed in different sizes. As a result, the identical RFsignals can be amplified in the different frequency bands. The simpledesign of the UWB LNA which is achieved by coupling the narrow band LNAscan expand and amplify the frequency band of the RF signal. Therefore,the UWB LNA can obtain both the low noise and the high gain advantagesof the conventional narrow band LNA.

It has been illustrated that the first and second amplification parts110 and 120, which are the narrow band amplifiers, are provided in apair. Note that more than two amplification parts can be designed. Whena plurality of narrow band amplifiers are employed, the frequency bandsof the amplified signal can be further expanded.

Although the first and second amplification parts 110 and 120 eachinclude the amplifying element and the pair of inductors, a narrow bandamplifier having other circuit elements, for example, amplifyingelements and resistors may also be used.

Although NMOS transistors are adapted as the amplifying elements M1 andM2 as shown in FIG. 5, other amplifying elements, for example, highelectron mobility transistors (HEMT) or bipolar transistors may beemployed.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A low noise amplifier (LNA) for ultra wide band (UWB), comprising:more than one pair of narrow band LNAs which is coupled in parallel, andreceives and amplifies identical RF signals in a predetermined frequencyband; and a load circuit which increases load impedance of the entirecircuit of the narrow band LNAs, wherein the amplifying element of eachnarrow band LNA is connected to a common-gate amplifying element whichforms a cascode amplifier together with the amplifying element.
 2. Thelow noise amplifier of claim 1, wherein the narrow band LNAs eachinclude an amplifying element and a pair of inductors which is connectedto a source and a gate of the amplifying element respectively.
 3. Thelow noise amplifier of claim 2, wherein the amplifying element is one ofan NMOS transistor, a high electron mobility transistor (HEMT), and abipolar transistor.
 4. The low noise amplifier of claim 2, wherein thecommon-gate amplifying element has a common gate input.
 5. The low noiseamplifier of claim 4, wherein the load circuit is positioned between thecommon-gate amplifying elements, and includes a load resistor, aninductor, and a capacitor that are connected in parallel.
 6. The lownoise amplifier of claim 5, further comprising: a buffer amplifyingelement which includes the load circuit located between a gate and adrain of the buffer amplifying element.
 7. A low noise amplifier (LNA)for ultra wide band (UWB), comprising: a plurality of narrow band LNAscoupled in parallel, wherein each narrow band LNA receives an identicalRF signal and amplifies the RF signal in a predetermined frequency band;and a load circuit which increases load impedance of the entire circuit,wherein the amplifying element of each narrow band LNA is connected to acommon-gate amplifying element which forms a cascode amplifier togetherwith the amplifying element.
 8. The low noise amplifier of claim 7,wherein each narrow band LNA comprises: an inductor connected to asource of the amplifying element; and an inductor connected to a gate ofthe amplifying element.
 9. The low noise amplifier of claim 8, whereinthe amplifying element is one of an NMOS transistor, a high electronmobility transistor (HEMT), and a bipolar transistor.
 10. The low noiseamplifier of claim 8, wherein each amplifying element is fabricated in adifferent size designed to amplify a different frequency band.
 11. Thelow noise amplifier of claim 8, wherein the inductor connected to thesource of the amplifying element and the inductor connected to the gateof the amplifying element are sized to provide a 50Ω input impedance foreach narrow band LNA.